President Barack Obama on Saturday rebuked Republican frontrunner Donald Trump for his incendiary language on the campaign trail.

At a Democratic party fundraising event in Dallas, Texas, Obama offered a blunt condemnation of the "divisiveness" fomented by Trump on the campaign trail, including his motto "Make America Great Again".

"We are great right now," Obama retorted, in remarks that came one day after skirmishes broke out at a scuttled Trump rally in Chicago.

"What the folks who are running for office should be focused on is how we can make it even better – not insults and schoolyard taunts and manufacturing facts, not divisiveness along the lines of race and faith. Certainly not violence against other Americans," Obama said.

Protests and scuffles

A Trump campaign event was cancelled in Chicago on Friday when throngs of protesters – many of them blacks and Latinos angered by Trump's anti-immigrant rhetoric – gathered at the venue, and in some cases brawling with the candidate's supporters. "We are not rapists," read one sign held by a protester inside the arena, referring to Trump's characterisation last year of Mexicans as rapists.

Trump has pledged to build a "great wall" at the border with Mexico to keep immigrants out, and earlier this week he said: "Islam hates us."

As the bombastic real estate mogul has edged further ahead of the once-crowded Republican field, Obama has sharpened his criticisms of him.

However, Trump was roundly defeated on Saturday by Florida Senator Marc Rubio for the Republican nomination in Washington DC and by Texas Senator Ted Cruz in Wyoming.

Big Tuesday coming up

Obama's ever-more direct criticism of Trump reflects a belief that the reality TV star may be the main thing standing between the Democrats and a third consecutive White House term.

Obama is expected to campaign vociferously for the eventual Democratic nominee, wielding his status as one of the country's most popular politicians to fire up the party faithful and make the case to young, black and Latino voters.

A Republican victory would throw much of Obama's legacy into doubt – from his landmark health care reforms to the detente with Cuba.

In other contests Saturday, Hillary Clinton won in the first ever Democratic Party caucus on the Northern Mariana Islands, a US region deep in the Pacific Ocean.

The biggest prizes will be on Tuesday, when primaries are held in five delegate-rich states – Florida, Illinois, Missouri, North Carolina and Ohio.